1. Field of the Invention
This invention relates generally to medical apparatus and methods, and more specifically to leads used to electrically and/or chemically modulate and monitor tissues of the brain.
Implanting medical devices such as probes or leads within the cranium is an increasingly important approach for treatment of diseases such as Parkinson's Disease, essential tremor and dystonia. Implants may be used to treat a wide array of disorders, such as depression, epilepsy, dystonia, obsessive compulsive disorder, obesity and chronic pain. Most of these devices interact with the brain by applying current through an electrode. In addition, infusion of drugs through a chronically implanted lead has been proposed in the medical literature either as a primary treatment, or as an adjunctive treatment to electrical stimulation, in patients with Alzheimer's and Parkinson's Diseases, among others.
Current implantable probes are typically configured as small diameter cylinders or tubes, with several circumferential metal rings near the distal tip, and an electrically passive central axial lumen. The metal rings are used to provide electrical stimulation, while the central axial lumen can be used to deliver the probe over a guidewire or stylet during the implantation procedure.
In most treatment protocols, a sequence of electrical pulses is applied to one or more conducting rings on the probe. Typically monopolar or bipolar stimulation of the conducting rings is used. In monopolar stimulation, a single circumferential ring is stimulated with a charge balanced biphasic electrical pulse, with a return path for the current at a remote site, such as a battery pack or control module. In bipolar stimulation, a combination of rings are stimulated with charge balanced biphasic electrical pulses of opposite polarity. Stimulation of conducting rings produce a field of action which is more or less symmetric about the probe, with some asymmetries arising because of anisotropy in the electrical properties of the adjacent neural or brain tissue.
A symmetrical electrical field about the probe axis is not always desirable. For example, when the probe is not implanted at the center of the modulation target or when the brain target is asymmetric or irregular in shape. Additionally, there are often neuronal domains near the targeted zone, which should not be modulated. Modulating non-target zones can lead to undesirable side effects, including somatic sensation, involuntary movement and impaired vision, among others.
It is desirable to not only modulate brain activity, but also to monitor it along with physiological and pathophysiological states. Monitoring obtains information on neuronal activity near the stimulation sites, including field potentials and extracellularly recorded action potentials. Such potentials may be observed on an ongoing basis, in the course of electrical stimulation for treatment, and in the course of special stimulation and response experiments designed to assess an individual's brain and the brain to electrode interface. Information obtained from monitoring at intervals may be used to control and adjust treatment on an ongoing, day-to-day basis by a patient, or in follow up visits to a health professional. Information obtained from monitoring may also be used to dynamically adjust the treatment by an automated control system or control algorithm, and by updating the parameters of a controller.
Monitoring at intervals can be used to track changes in the brain response to stimulation as a function of stimulus magnitude. Clinical decisions can be based upon estimated parameters, such as the threshold stimulus level which barely generates a response, and the stimulus level which just saturates the observed response. The shape of the stimulus response function, for example whether it is concave up, concave down, or linear, may also inform adjustments to treatment. The dynamic range from threshold to saturation measured near the stimulation site may directly correspond to the dynamic range of clinical effect, or it may be correlated with it. In either case, the locally measured dynamic range gives information which can accelerate the initial fitting and guide ongoing adjustments in treatment protocol. Brain plasticity in response to treatment may be tracked by changes in the dynamic range.
Consider the application of monitoring at intervals to the treatment of Parkinson's disease. It is well known that the beneficial effects of electrical stimulation to Parkinson's patients do not appear for several minutes or hours after the stimulation protocol is initiated. If the protocol is discontinued during sleep and resumed at waking, the beneficial effects of treatment may not appear again for many hours. Monitoring at intervals offers the opportunity to track changes in the response to stimulation, so that stimulation can be applied during one protocol in order to bring about the beneficial effects, and under another more conservative protocol in order to just maintain the beneficial effects. Such a strategy would conserve battery power, and could also reduce side effects.
By monitoring from moment to moment, a modulatory treatment can be dynamically synchronized with natural brain rhythms upon an observed pathological or normal physiological state, or controlled by an automatic control system or control algorithm.
Most procedures currently performed monitor patient motions, behaviors, or brain activity at a site remote from the site of an electrically stimulating probe, and this information is used to adjust brain stimulation parameters. Parameters are adjusted on a short time scale, to generate a desired effect and minimize side effects, and on a longer time scale, to account for brain plasticity. Brain plasticity is due to an adaptive response by the brain to an intervention and it is well known that ongoing responses by the brain to an intervention such as modulating therapy often differ from the initial response. Useful information may also be obtained by monitoring electrical potentials near the site of electrical stimulation and therefore it would be desirable to monitor brain activity at the locus of electrical stimulation. Monitoring allows the course of the disease and healing processes to be evaluated along with the prognosis for various treatment options.
For these reasons as well as others, it would be desirable to provide improved probes for modulating and monitoring tissues such as the brain. It would be particularly desirable to provide an efficient design for generating a directed electrical field that may be steered towards the intended target, and/or away from other brain areas. It is also desirable to provide a probe with an efficient number and size of electrodes as well as connector leads, that integrates both electrical recording and stimulating or modulating capabilities, where the information from recordings is obtained close to the treatment site and can be used to define the stimulating protocol. The protocol can then be adapted either statically or dynamically and as the disease state changes, the therapy can also be adjusted. Recording and monitoring of brain electrical activity is also used to determine when the stimulation protocol is applied or whether it should be reserved for times when it is more effective, thereby helping to conserve power.
2. Description of the Background Art
Prior patents and publications describing brain modulating probes and methods include: U.S. Publication Nos. 2006/0047325; 2006/0004422; 2005/0015130; 2004/0039434 and U.S. Pat. Nos. 7,051,419; 7,047,082; 7,006,872; 6,094,598; 6,038,480; 6,011,996; 5,843,148; and 5,716,377.